1. Field of the Invention
The present invention relates to an arrangement for extracting a DC or low frequency signal superimposed on a RF signal fed into a filter. The invention further relates to the filter that comprises a filter housing and a filter lid.
2. Description of the Related Art
A DC voltage and a low frequency communication signal are often superimposed on a RF signal fed into a filter for mobile communication. The DC voltage may be used to drive a low noise amplifier (LNA) in the filter and the low frequency communication signal contains information that is used internally in the filter, sent from the filter or passed through the filter, together or not with the DC voltage, without being distorted by the transfer function of the filter.
To be able to realize this, the DC voltage, together with the low frequency communication signal, have to be removed from the input signal before the RF signal enters the filter structure, and, if desired, the DC voltage and/or the low frequency communication signal may be added to the output of the filter.
Several solutions have been proposed during the years and FIGS. 1 and 2 disclose solutions which are described in more detail below. The principal function of the arrangement for extracting the DC voltage and the low frequency communication signal comprises a low pass filter (LP filter) arranged in parallel with an input coupling rod or first resonator. Both solutions described in FIGS. 1 and 2 have similar drawbacks, since they are difficult to produce in a reliable and stable way. There are risks for passive inter modulation (PIM) and it is difficult to control the tolerances of the structure which limits performance and makes the solutions described in connection with FIGS. 1 and 2 expensive to manufacture. Furthermore, RF tends to leak into the DC voltage and the low frequency communication signal due to the structure of the LP filter.
A structure similar to the claimed invention is disclosed in U.S. Pat. No. 5,023,579 by Salvatore et al., that describes an integrated band pass/low filter where the first and last resonators are coupled to associating connectors. Low pass filters are positioned within the first and last resonators and the RF signal fed into the resonators are subject to low pass filtration thus forming a band pass filter for the RF signal. Thus, the RF signal is subject to low pass filtering which is not the object of the invention.